I found an investor. Is that fair?

That's how these things work. I'd go for a more reasonable deal unless he is buying like EVERYTHING. After a while you can attempt to buy his half of the business from him or you could go a different route by trying to get a couple of small investors that can invest an equal amount and slowly by them off so it's less money for each part of the business. Like if it were just you and that guy and he owned half of the business and it was valued at 100,000 dollars you'd have to offer him AT LEAST 50,000 dollars for it, but if it were to have like 3 investors still valued at 100,000 dollars, you'd own 25% of the business, only making 25% at first then eventually buying out the other pieces of rights to the business. And as you own more of the business you receive that much more profit.